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[SR-1581] Can not use protocol to fulfill associatedtype requirement where associatedType has protocol constraint #44190

swift-ci opened this issue May 21, 2016 · 2 comments


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@swift-ci swift-ci commented May 21, 2016

Previous ID SR-1581
Radar None
Original Reporter vishal.panwar (JIRA User)
Type Bug
Status Resolved
Resolution Duplicate
Additional Detail from JIRA
Votes 10
Component/s Compiler
Labels Bug
Assignee None
Priority Medium

md5: d2a9b745ccd1923da6ccec087b0df3f9


  • SR-55 non-@objc protocol existentials do not conform to their own protocol type

is duplicated by:

  • SR-11340 Compilation error when there shouldn't be one

relates to:

  • SR-55 non-@objc protocol existentials do not conform to their own protocol type
  • SR-2020 Protocol composition doesn't conform to associated type requirement

Issue Description:

The following code sample does not compile.

protocol Bar {   

protocol Buz: Bar {   

protocol Foo {
    associatedtype SomeType: Bar
    func someOperation(someType: SomeType)

struct Quack: Foo {
    func someOperation(someType: Buz) {

It throws error:

Inferred type 'Buz' (by matching requirement 'someOperation') is invalid: does not conform to 'Bar'

I am not sure if we are only allowed to use a concrete type for associatedtype with protocol constraint. I don't see any reason for this restriction. If we remove the protocol constraint from associatedtype, then it compiles with no error.

Update 2016/12/12: I am using below workaround to tackle above bug. This seems more logical and makes me feel like its not a bug.

struct Quack<T>: Foo where T: Buz {
    func someOperation(someType: T) {

// Usage
struct SomeBuz: Buz {}
let quack: Quack<SomeBuz> = Quack<SomeBuz>()

The reason this makes more sense to me is that the compiler now has more context on the actual type.

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@belkadan belkadan commented May 23, 2016

Protocols don't always "conform to themselves", but this configuration seems safe. @jckarter?

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@karwa karwa commented Jun 28, 2016

+1 for this. The thing that we want to express is safe; if there are nuances with the current syntax we should devise a way to make it possible.

protocol A {}
protocol SubA : A {}

protocol B { associatedtype TypeOfA : A }

struct MyB : B {     // ERROR: `MyB` does not conform to `B`
    typealias TypeOfA = SubA

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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